Chemical vapor deposition is a well known process for creating deposits of various composition, particularly metallic nitrides and carbides by means of passing gaseous mixture over a heated substrate causing the gaseous mixture to react and produce the desired substance on the substrate. Such a process is described in copending Canadian patent application No. 228,901 filed June 9, 1975 and assigned to the assignee of the instant application, which application discloses a process for depositing barrier layers on filaments.
Filaments have been proposed in the past for use in reinforcing composite structures and as described in the art, these filaments may be immersed in an alloy member and contribute to the high temperature strength of the resultant composite product. However, many of these filaments, for example, tungsten, silicon carbide, when immersed in such alloys, react with components of the alloys and, if exposed for long periods of time at high temperature, degrade due to migration of alloy constituents into the fibers. Certain coating materials will reduce this migration as described in Canadian application No. 228,901. It has been found however, that the process described in the foregoing application does not produce layers sufficiently thick for some applications, particularly when efforts are made to apply protective coatings on silicon carbide.
It appears that one of the causes of the poor rate of deposition of the barrier material on silicon carbide substrates, is the undesirable temperature distribution along the length of the substrate. In cases where the substrate is a fiber and where the heat is supplied by flowing electrical current through the fiber, it is evident that the temperature is a function of the i.sup.2 r losses in the filament. It will also be evident that as coating proceeds, the fiber becomes effectively thicker and the i.sup.2 r losses decrease Also, in the case where the barrier material produced on the surface of the fiber has a lower resistance per cube than the material of the fiber, then the thicker the barrier layer, the less the i.sup.2 r losses. The sum of these effects therefore, is to decrease the temperature of the coated portion. In order that the rate of coating be uniform along the length of the substrate as it travels through the coating chamber in a continuous coating process, it would seem desirable that a suitable temperature be maintained substantially uniform along the whole length. In addition, the substrate obviously has limits to its maximum permissible temperature and if the temperature is not uniform along the length of the substrate, the maximum temperature location will operate as the limiting temperature for the system.